Abstract:

A silica glass crucible having a sidewall portion and a bottom portion is
provided with a first synthetic silica glass layer constituting an inner
layer at least in the sidewall portion, a second synthetic silica glass
layer constituting an inner layer at least in a region including a center
of the bottom portion, and a natural silica glass layer constituting an
outer layer in the sidewall portion and the bottom portion. A melting
rate of the second synthetic silica glass layer with respect to a silicon
melt is higher than that of the first synthetic silica glass layer. An
aluminum concentration of the second synthetic silica glass layer is
higher than that of the first synthetic silica glass layer.

Claims:

1. A silica glass crucible having a sidewall portion and a bottom portion,
comprising:a first synthetic silica glass layer constituting an inner
layer at least in the sidewall portion; anda second synthetic silica
glass layer constituting an inner layer at least in a region including a
center of the bottom portion, whereina melting rate of the second
synthetic silica glass layer with respect to a silicon melt is higher
than that of the first synthetic silica glass layer.

2. The silica glass crucible as claimed in claim 1, further comprising a
natural silica glass layer constituting an outer layer in the sidewall
portion and the bottom portion.

3. The silica glass crucible as claimed in claim 2, whereinthe first
synthetic silica glass layer is formed on an inner surface including a
sidewall portion and a bottom portion of the natural silica glass layer,
andthe second synthetic silica glass layer is formed on an inner surface
including a center of a bottom portion of the first synthetic silica
glass layer.

4. The silica glass crucible as claimed in claim 3, wherein a thickness of
the second synthetic silica glass layer is smaller at locations farther
away from the center of the bottom portion.

5. The silica glass crucible as claimed in claim 4, wherein the thickness
of the second synthetic silica glass layer is 30 μm to 200 μm at
the center of the bottom portion.

6. The silica glass crucible as claimed in claim 1, wherein the melting
rate of the second synthetic silica glass layer with respect to the
silicon melt is two to three times of the melting rate of the first
synthetic silica glass layer with respect to the silicon melt.

7. The silica glass crucible as claimed in claim 1, wherein an aluminum
concentration of the second synthetic silica glass layer is higher than
that of the first synthetic silica glass layer.

8. The silica glass crucible as claimed in claim 7, wherein the aluminum
concentration of the second synthetic silica glass layer is 2 ppm to 20
ppm.

9. The silica glass crucible as claimed in claim 8, wherein an aluminum
concentration of the first synthetic silica glass layer is 0.2 ppm or
less.

10. A silica glass crucible including a sidewall portion and a bottom
portion, in which at least an inner layer of the crucible comprises a
synthetic silica glass layer, whereinan aluminum concentration of the
synthetic silica glass layer in a region including a center of the bottom
portion is higher than that in the sidewall portion.

11. The silica glass crucible as claimed in claim 10, wherein the aluminum
concentration of the synthetic silica glass layer provided in the region
including the center of the bottom portion is 2 ppm to 20 ppm.

12. The silica glass crucible as claimed in claim 11, wherein the aluminum
concentration of the synthetic silica glass layer provided in the
sidewall portion is 0.2 ppm or less.

13. The silica glass crucible as claimed in claim 10, wherein a thickness
of the synthetic silica glass layer having the higher aluminum
concentration than that of the sidewall portion is 30 μm to 200 μm
at the center of the bottom portion.

14. The silica glass crucible as claimed in claim 13, wherein the
thickness of the synthetic silica glass layer having the higher aluminum
concentration than that of the sidewall portion is smaller at locations
farther away from the center of the bottom portion.

15. A silica glass crucible used for growing a silicon single crystal,
wherein a high temperature melting rate of a bottom central portion of
the crucible including a projection plane of a silicon single crystal is
higher than that of a peripheral of the central portion.

16. The silica glass crucible as claimed in claim 15, wherein the high
temperature melting rate of the bottom central portion of the crucible
including the projection plane of the silicon single crystal is two to
three times of the high temperature melting rate of the periphery of the
central portion.

17. The silica glass crucible as claimed in claim 15, wherein an inner
surface layer of the bottom central portion is provided with a synthetic
silica layer including aluminum, whereby the high temperature melting
rate of a bottom central portion is higher than that of the peripheral of
the central portion.

18. The silica glass crucible as claimed in claim 17, wherein an aluminum
concentration of the synthetic silica layer is 2 ppm to 20 ppm.

19. The silica glass crucible as claimed in claim 17, wherein a thickness
at a center of the synthetic silica layer is 30 μm to 200 μm.

20. The silica glass crucible as claimed in claim 15, whereinan outer
layer of the crucible is a natural silica layer,an inner layer of the
crucible is a synthetic silica layer,a synthetic silica layer including
aluminum is laminated on a bottom central portion of the synthetic silica
layer, anda flat inner surface layer is formed without a step with
respect to an outer range of the central portion.

Description:

TECHNICAL FIELD

[0001]The present invention relates to a silica glass crucible used for
growing a silicon single crystal, and more particularly relates to a
silica glass crucible in which bubbles adhering to an inner surface of
the crucible are suppressed in the growing step for the silicon single
crystal.

BACKGROUND OF THE INVENTION

[0002]A silica glass crucible is used for growing a silicon single
crystal. For example, the silicon single crystal is produced by heating
and melting polycrystalline silicon charged into the silica grass
crucible to obtain silicon melt, dipping a seed crystal into the silicon
melt, and pulling the seed crystal upwards. During the growth of the
silicon single crystal, if bubbles adhering to an inner surface of the
silica glass crucible or generated float up and adhere to the silicon
single crystal, the bubbles are taken into the silicon single crystal to
form air pockets.

[0003]In recent years, influence exerted by the air pocket in a silicon
wafer on a semiconductor device has been quite serious. The influence of
the air pockets is varied depending upon their magnitudes, the number of
the air pockets, locations where the air pockets are generated, and the
type of a semiconductor device. However, since a size of the air pocket
is very larger than that of COP (Crystal Originated Particle), no device
can be formed in a space of the air pocket. Especially when the number of
air pockets in a wafer is high, yield of the semiconductor device is
considerably deteriorated and thus, the wafer itself must be discarded.
It is therefore necessary to reduce a possibility of the air pockets
existing in the wafer as close to zero as possible.

[0004]Japanese Patent Application Laid-Open No. 2001-519752 shows that Ar
components of bubbles included in a crucible are taken into a silicon
single crystal to form air pockets. Japanese Patent No. 3046545 shows
that Ar gas which was taken in when polycrystalline silicon is melted
adheres to an inner surface of a crucible, and when a silicon crystal is
grown, the Ar gas is taken into the silicon single crystal. As a cause of
air pockets, in addition to the Ar gas, SiO gas generated by a reaction
between a silica glass crucible and a silicon melt may be taken in.

SUMMARY OF THE INVENTION

[0005]The present invention provides a silica glass crucible in which the
air pockets are suppressed. The present invention is based on findings
that a flaw in an inner surface of a crucible is one of causes of air
pockets taken into a silicon single crystal. If there is a flaw on the
inner surface of the crucible, SiO gas is generated from this flaw as a
nucleus, the SiO gas then moves upwards in the silicon melt, and the gas
is taken into the silicon single crystal. Flaws on the inner surface of
the crucible are generated when the crucible is produced or when the
polycrystalline silicon is charged or melted.

[0006]The present invention provides a silica glass crucible in which
flaws on the inner surface of the crucible are eliminated before a
silicon crystal is grown, thereby restraining SiO gas from being
generated, and preventing air pockets in a silicon single crystal.

[0007]To solve the conventional problem, the present invention provides
with a silica glass crucible having a sidewall portion and a bottom
portion, comprising a first synthetic silica glass layer constituting an
inner layer at least in the sidewall portion, and a second synthetic
silica glass layer constituting an inner layer at least in a region
including a center of the bottom portion, wherein a melting rate of the
second synthetic silica glass layer with respect to a silicon melt is
higher than that of the first synthetic silica glass layer.

[0008]In a preferred embodiment of the present invention, the high
temperature melting rate of the bottom central portion of the crucible
including the projection plane of the silicon single crystal is two to
three times of the high temperature melting rate of the periphery of the
central portion.

[0009]In a preferred embodiment of the present invention, an inner surface
layer of the bottom central portion is provided with a synthetic silica
layer including aluminum, whereby the high temperature melting rate of a
bottom central portion is higher than that of the peripheral of the
central portion.

[0010]In a preferred embodiment of the present invention, the silica glass
crucible as claimed in claim 17, wherein an aluminum concentration of the
synthetic silica layer is 2 ppm to 20 ppm.

[0011]In a preferred embodiment of the present invention, a thickness at a
center of the synthetic silica layer is 30 μm to 200 μm.

[0012]In a preferred embodiment of the present invention, an outer layer
of the crucible is a natural silica layer, an inner layer of the crucible
is a synthetic silica layer, a synthetic silica layer including aluminum
is laminated on a bottom central portion of the synthetic silica layer,
and a flat inner surface layer is formed without a step with respect to
an outer range of the central portion.

[0013]According to the silica glass crucible of the present invention, the
high temperature melting rate of the central portion of the crucible
bottom surface including the projection plane of the silicon single
crystal is higher than a high temperature melting rate on an outer side
of the central portion, and melting of the inner surface of the central
portion is promoted at high temperatures. Therefore, flaws are eliminated
and generation of SiO gas is suppressed. That is, even if there is a fine
flaw on the bottom surface of the crucible, this flaw can be eliminated
for a short time period from a polysilicon melting step to a start of
growth of a silicon crystal. Therefore, when the growing step is
practically started, the bottom surface of the crucible can be brought
into a fine flaw-free state. Meanwhile, it was found that the SiO gas
generated on the inner surface of the crucible substantially vertically
moved upwards in the silicon melt, and was not affected by convection of
the silicon melt. Therefore, when flaws on the inner surface of the
central portion of the crucible bottom surface including the projection
plane of the silicon single crystal are eliminated, it is possible to
effectively prevent the SiO gas from being taken into the silicon single
crystal, and to prevent air pockets from being generated in the silicon
single crystal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]The above and other objects, features and advantages of this
invention will become more apparent by reference to the following
detailed description of the invention taken in conjunction with the
accompanying drawings, wherein:

[0015]FIG. 1 is a sectional explanatory diagram showing a relation between
projection planes of a crucible and a silicon single crystal; and

[0016]FIG. 2 is a sectional explanatory diagram showing a structure of the
crucible.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0017]Preferred embodiments of the present invention will now be described
in detail hereinafter with reference to the accompanying drawings.

[0018]A silica glass crucible of the present invention is used for growing
a silicon single crystal, and is characterized in that a high temperature
melting rate of a central portion of a bottom surface of the crucible
including a projection plane of the silicon single crystal is higher than
that of a periphery of the central portion. The high temperature melting
rate means a melting rate with respect to a silicon melt. This is because
according to the invention, flaws on an inner surface of a silica glass
crucible are eliminated in a state where the crucible is filled with a
silicon melt. The silicon melt in the silica glass crucible is set to
about 1500° C.

[0019]The silica glass crucible of the invention is adjusted such that the
high temperature melting rate (melting rate with respect to the silicon
melt) of the central portion of the bottom surface of the crucible
including the projection plane of the silicon single crystal becomes
higher than that of the periphery of the central portion. In the
following description, the central portion of the bottom surface of the
crucible including the projection plane of the silicon single crystal is
simply called a bottom central portion. FIG. 1 shows this state. As shown
in FIG. 1, a projection plane of a silicon single crystal 2 is an axial
projection plane, and the bottom central portion is a range including a
radial cross section S of the silicon single crystal 2 around a rotation
axis of a crucible 1. The bottom central portion matches with the silicon
crystal cross section S or is slightly wider than the cross section.

[0020]The projection plane of the silicon single crystal 2 is not uniquely
determined from a shape and a size of the silica glass crucible 1, but
the projection plane largely depends on an opening diameter of the silica
glass crucible 1. When the opening diameter of the crucible is so smaller
than the diameter of the silicon single crystal 2, it becomes difficult
to control crystal quality such as oxygen concentration and oxygen
in-plane distribution of the silicon crystal. On the other hand, when the
opening diameter is so great, cost is increased because it is necessary
to increase the device and members in size. Therefore, when the opening
diameter of the crucible is defined as Ro and the diameter of the
silicon single crystal 2 is defined as Rs, it is normal that Rs
is set to 0.3Ro to 0.6Ro.

[0021]The SiO gas on the inner surface of the crucible substantially
vertically moves up in the silicon melt. Therefore, when the SiO gas is
restrained from being generated at the bottom central portion, it is
possible to effectively prevent the SiO gas from being taken into the
silicon single crystal. It is not preferable to increase the melting rate
of the entire inner surface of the crucible, because eluted impurities
and crystallized silica pieces peeled off are increased, which lowers
yield of the silicon single crystal.

[0022]In the silica glass crucible of the present invention, the high
temperature melting rate of the bottom central portion of the crucible
including the projection plane of the silicon single crystal is
preferably about two to three times of the high temperature melting rate
of the periphery of the central portion. Specifically, the melting rate
of the bottom central portion is 10 to 15 μm/hr at 1500° C.,
and the melting rate at a location outward of the bottom central portion
is 3 to 6 μm/hr at 1500° C., for example. When the melting rate
of the outward portion is reduced, it is possible to suppress excessive
melting of the inner surface of the crucible. When the melting rate of
the bottom central portion is increased, it is possible to promote the
melting of the bottom central portion, to eliminate flaws on the inner
surface and suppress generation of the SiO gas.

[0023]When the silicon single crystal is grown, it takes about 10 hours or
more to melt the polycrystalline silicon charged into the crucible. At
the time of melting, when the polycrystalline silicon charged into the
crucible is partially melted and a non-melted portion crumbles, flaws may
be generated on the inner surface of the crucible. According to the
silica glass crucible of the invention, however, the melting of the inner
surface of the crucible is promoted and flaws are eliminated at the time
of melting. Therefore, flaws on the inner surface generated at the time
of melting are also eliminated, and it is possible to effectively prevent
the SiO gas from being taken into the silicon single crystal.

[0024]When the melting rate of the bottom central portion is smaller than
10 μm/hr, it becomes difficult to sufficiently eliminate flaws on the
inner surface before the pulling operation. It is not preferable that the
melting rate of the bottom central portion is higher than 15 μm/hr,
because the inner surface of the crucible is excessively melted, which
causes a worry that strength is deficient. The melting rate of the
location outward of the bottom central portion (periphery of the bottom
central portion) can be equal to the melting rate of normal silica glass
(about 5 μm/hr).

[0025]In the silica glass crucible of the present invention, the high
temperature melting rate of the inner surface of the bottom central
portion of the crucible is increased by including aluminum in the central
portion, for example. It is generally considered that when the aluminum
concentration in silica glass is increased, the glass has increased
viscosity and it becomes less prone to be melted. For example, an
approach is known that changes in melting amount are suppressed by
increasing an aluminum content in silica glass (International Publication
No. WO 2004-106247). According to the findings of the present inventors,
the melting of silica glass can be promoted by including aluminum in
silica glass.

[0026]An appropriate aluminum content included in the bottom central
portion of the inner surface layer of the crucible is in a range of 2 to
20 ppm, and preferably in a range of 2 to 5 ppm. When the aluminum
content is less than 2 ppm, the melting rate is not enhanced
sufficiently, and when the content exceeds 20 ppm, the silicon crystal
yield is lowered. When 2 to 20 ppm aluminum is included, the melting rate
of the bottom central portion can be adjusted to about 10 to 15 μm/hr
at 1500° C.

[0027]The silica glass crucible of the invention can be formed into a
structure shown in FIG. 2. In FIG. 2, an outer layer of a silica glass
crucible 10 is a natural silica layer 11. An inner layer of the crucible
is a synthetic silica layer 12, and the total content of impurities such
as alkali metal, alkali earth metal, iron, aluminum, boron, and
phosphorus is 0.2 ppm or less. A synthetic silica layer 13 including
aluminum is laminated on a bottom central portion of the synthetic silica
layer 12, and the synthetic silica layer 13 including aluminum has a flat
inner surface having no step with respect to a periphery (a corner and a
sidewall) 14 of the central portion.

[0028]The structure of the silica glass crucible 10 will be explained more
specifically. The sidewall portion has a two-layer structure including
the natural silica layer 11 and the synthetic silica layer 12, and a
region thereof including a center of its bottom has a three-layer
structure including the natural silica layer 11, the synthetic silica
layer 12, and the synthetic silica layer 13 including aluminum. That is,
the outer layer is all constituted of the natural silica layer 11, the
synthetic silica layer 12 is formed on the entire inner surface of the
natural silica layer 11 including the sidewall portion and the bottom,
and the synthetic silica layer 13 including aluminum is selectively
formed on the inner surface including the bottom center of the synthetic
silica layer 12.

[0029]Generally, a concentration of metal impurities included in the
natural silica layer 11 is higher than that in the synthetic silica layer
12, and OH group concentration of the natural silica layer 11 is lower
than that in the synthetic silica layer 12. The natural silica layer 11
has different characteristics from those of the synthetic silica layer 12
also in X-ray diffraction measurement. Determination of the natural
silica layer 11 and the synthetic silica layer 12 needs not to be
performed based on one determination element, but to be performed
comprehensively based on a plurality of determination elements.

[0030]According to the silica glass crucible 10 shown in FIG. 2, the
synthetic silica layer 12 constitutes the inner layer of the sidewall
portion, and the synthetic silica layer 13 including aluminum constitutes
the inner layer of the region including the bottom center. The melting
rate of the synthetic silica layer 13 including aluminum with respect to
the silicon melt is higher than that of the synthetic silica layer 12.
More specifically, it is preferable that the melting rate of the
synthetic silica layer 13 including aluminum with respect to the silicon
melt be two to three times of the melting rate of the synthetic silica
layer 12 with respect to the silicon melt.

[0031]As described above, the melting rate of the silica grass with
respect to the silicon melt can be controlled by the aluminum
concentration. More specifically, the melting rate with respect to the
silicon melt becomes higher with increasing the aluminum concentration.
Therefore, it is only necessary to set the aluminum concentration of the
synthetic silica layer 13 including aluminum higher than that of the
synthetic silica layer 12. Specifically, it is only necessary that the
aluminum concentration of the synthetic silica layer 13 including
aluminum is set to 2 to 20 ppm, and that the aluminum concentration of
the synthetic silica layer 12 is set to 0.2 ppm or less. The reason
thereof is as described above.

[0032]It is preferable that the thickness of the synthetic silica layer 13
including aluminum be in a range of 30 to 200 μm/hr at the bottom
center. This is because the depth of flaws generated when the
polycrystalline silicon is charged and when the polycrystalline silicon
is melted is about 50 μm to 100 μm. Therefore, an appropriate
center layer thickness of the synthetic silica layer 13 including
aluminum at the bottom central portion is in a range of 30 μm to 200
μm, and preferably, in a range of 50 μm to 120 μm. When the
layer thickness is less than 30 μm, flaws on the inner surface are not
sufficiently eliminated in some cases. When the layer thickness is
greater than 200 μm, the effect is substantially the same. It is
preferable that the thickness of the synthetic silica layer 13 including
aluminum be smaller at locations farther away from the bottom center.
With this structure, even if the synthetic silica layer 13 including
aluminum is provided, no step is generated in the inner surface of the
silica glass crucible 10 and thus, various inconveniences caused by the
step can be prevented.

[0033]As described above, the projection plane of the silicon single
crystal is not uniquely determined from the shape and size of the silica
glass crucible, and when the opening diameter of the crucible is defined
as Ro and the diameter of the silicon single crystal is defined as
Rs, it is normal that Rs is set to 0.3Ro to 0.6Ro.
Taking this point into consideration, when a diameter of a planar region
of the synthetic silica layer 13 including aluminum is defined as
R1, it is preferable that R1 be 0.3Ro or more and
0.6Ro or less. When the diameter R1 of the planar region of the
synthetic silica layer 13 including aluminum is smaller than 0.3Ro,
the projection plane of the silicon single crystal cannot be covered, and
the probability that bubbles of the SiO gas generated from the synthetic
silica layer 12 are taken into the silicon single crystal is increased.
When the diameter R1 of the planar region of the synthetic silica
layer 13 including aluminum exceeds 0.6Ro, the projection plane of
the silicon single crystal can reliably be covered. However, impurities
eluting into the silicon melt may increase, and the yield of the silicon
single crystal may decrease.

[0034]The diameter R1 of the planar region of the synthetic silica
layer 13 including aluminum will be explained specifically. For example,
when a silicon single crystal having a diameter of about 300 mm is to be
grown using a silica glass crucible of 32 inches (its opening diameter
RO is about 800 mm), the lower limit of a diameter R1 of a
circular region of the synthetic silica layer 13 including aluminum
formed at the bottom of the crucible is 0.3Ro=240 mm, and the upper
limit thereof is 0.6Ro=480 mm. Usually, the 32-inch crucible is used
for growing the silicon single crystal having the diameter of about 300
mm, and it is preferable that the diameter R1 of the planar region
of the synthetic silica layer 13 including aluminum in this case be about
300 mm. This value satisfies the condition of not less than 240 mm and
not more than 480 mm. When the diameter R1 of the planar region of
the synthetic silica layer 13 including aluminum is not less than
0.3Ro and not more than 0.6Ro, it is possible to effectively
suppress the generation of the SiO gas bubbles which can be taken into
the silicon single crystal being grown without hardly lowering the
silicon single crystal yield.

[0035]As described above, the SiO gas bubbles substantially vertically
float. However, it is also conceived that bubbles generated outward of
the projection plane of the silicon single crystal being grown (region
where the synthetic silica layer 13 including aluminum is not formed)
float while slightly shifting horizontally for any reason and, as a
result, the bubbles are taken into the silicon single crystal. Because
positions of such bubbles are near the outer periphery of the silicon
single crystal and the outer peripheral area of the silicon single
crystal is ground later as an unnecessary portion, there is no problem
even if the bubbles are taken in the silicon single crystal.

[0036]The present invention is in no way limited to the aforementioned
embodiments, but rather various modifications are possible within the
scope of the invention as recited in the claims, and naturally these
modifications are included within the scope of the invention.

Examples

[0037]Silica glass crucibles (opening diameter thereof is 32 inches) of
structures shown in FIG. 2 having a natural silica outer layer and a
synthetic silica inner layer were produced under conditions shown in
Table 1 or 2, and silicon single crystals having a diameter of about 300
mm were pulled-up with the crucibles.

[0038]after pulling the silicon single crystal, wafers having thickness of
about 1 mm were produced by slicing the silicon single crystal ingot with
a wire-saw, and the surface of the wafers were polished. Next, the air
pocket generation ratio of each polished wafers was measured. the are
pocket generation ratio is the value obtained by dividing total number of
the air pocket including all wafers obtained from one silicon single
crystal by total number of the wafers. A particle counter was used for
counting the number of the air pocket exposed on the surface of the
polished wafer.

[0039]The yield of the silicon single crystal was also measured. The yield
of the silicon single crystal is defined as the weight ratio of the
silicon single crystal to the polycrystalline silicon material. Since all
silicon melt in the crucible was not consumed and only straight body part
of the ingot excepting the top and bottom part is the object to be
measured. Thus, the yield of the silicon single crystal is under 100%
even if the silicon single crystal is fully pulled-up, and more than 80%
of the yield will be a good result. Results thereof are shown in Tables 1
and 2.

[0040]Air pockets were not generated in any of crucibles of examples A1 to
A4, and each yield of the silicon single crystal were high. In a crucible
of a comparative example C1, a generation ratio of air pockets was high.
Also in a crucible of a comparative example C2, the generation ratio of
air pockets was not sufficiently reduced. In a crucible of a comparative
example C3, although no air pocket was generated, yield of the silicon
single crystal was considerably lowered. According to the results shown
in Table 1, it was confirmed that the aluminum content of the synthetic
silica layer provided at the bottom central portion is preferably in a
range of about 2 ppm to 20 ppm.

[0041]In any of crucibles of examples B1 to B4, no air pocket was
generated, and each yield of the silicon single crystal were high. In a
crucible of a comparative example C4, although no air pocket was
generated, yield of the silicon crystal was considerably lowered, and in
a crucible of a comparative example C5, a generation ratio of air pockets
was high. According to the results in Table 2, it was confirmed that the
thickness at the center of the synthetic silica layer including aluminum
provided at the bottom central portion is preferably in a range of 30
μm to 200 μm.